This invention relates to optical couplers and more particularly to an optical coupler for a graded index fiber.
The graded index fiber has been in existence for a number of years and has a core with a refractive index profile that is radially symmetric. The refractive index profile of the core is approximately parabolic in shape being highest at the center of the core and descreasing parabolically until it matches the cladding refractive index at the core clad interface. Light which is propagated into the core of any fiber and therefore a graded index fiber at an angle less than the critical acceptance angle (numerical aperture NA) is reflected internally upon striking the core-clad interface and therefore continues to propagate within the fiber core. Graded index fibers minimize propagation delay between various modes and therefore can handle large bandwidths as compared to step index fibers.
In any event, it has been a problem in the prior art to apply high and low order modes to a graded index fiber with small crosstalk. Essentially, as indicated, a graded index fiber can handle a large number of modes. Since both the high and low order modes are internally reflected within the fiber, they will interfere with each other and produce crosstalk or coupling between the two modes.
It is therefore an object of the present invention to excite high and low order modes within a graded index fiber, while further providing a relatively insignificant amount of crosstalk.